Method of forming an adherent film of magnesium oxide



METHOD or FORMING AN ADHERENT FILM OF MAGNESIUM OXIDE.

Lydia A. Snchofl, Asbury Park, N. 1., assignor m ne" United States of America as represented by t l1'e Se'c'retary of the Army No Drawing. Application October 2, 1952, Serial No.312,873

j Claims. (CL 115 212 I Granted under Title 35, U. s. cone- 952 at; aa-

The invention described herein may be manufactured and used by or for the'Government-for governmental purposes, without the payment of any royalty thereon;

This invention relates to coating methods" and more particularly to a method for providing avery thin," ad

herent coating of magnesium oxide.

Most of the known processes for providing magnesium oxide: coatings tend to produce films which cannot :be accurately controlledv forspecific thicknesses and are not sufliciently adherent to the underlying surfacebe-i ing coated. This difficulty is encountered" especially where'extremely thin coatings are desired. .js' In certain instances such as in the insulating coating of magnetic cores, particularly those produced of'ligh't gauge material, 0.002" to 0.0001" in thickness, 'the u'se of such light gauge material is dictated by the need to minimize the eddy current losses which reduce the' usefulness of magnetic cores' in applications employing high frequencies. The term high frequencies cannotbe clearly defined, since the amount of eddy currentjlos ses depends on the resistivity'of the material, its thickness, and its magnetic properties. For materials haVi-n'ghigh permeability and low resistivity, the gauge to be used at a certain frequency will have to be lighter than for a material having low permeability and high resistivity. In terms of frequency, it can be shown that for one ma terial a frequency of 60 C. P. S. (cycles per second) may produce as much eddy current losses as a frequency of 1,000 C. P. S. or even more does for another material ofthe -sam'e gauge. High frequencies will therefore be frequencies at which eddy current losses begin.

to become pronounced, and, for thin gauge materials they may range from 60 cycles upward to the megacycle range. 1 Obviously, the purpose of the lamination into li'ght gauge assemblies would be defeated if the individual laminations would be allowed to remain in electrical contact, thereby providing the same path for eddy currents as a solid block. The insulation of the individual laminations in a stack, or of the adjacent convolutions of a wound core is therefore of great technical importance for the application of magnetic materials at high frequencies;

The requirement of high insulation resistance between laminations must be reconciled with the obvious concomitant requirement that the space factor of the laminated stack, i. e., the percentage of the cross-section 2,796,354 Patented June 18 1957 'n eal of the stacked laminations or wound cores which may involve the exposure of the coresto temperatures of 1000 C, to 1200 C. T

The main requirements to be met by a satisfactory insulating coating for magnetic cores can therefore be summarized as being uniformly high electrical resistivity, minimum thickness, and high temperature stability. 1 Be- I sides these properties, the coating should also withstand somedegree of bending or handling of the'coated parts. It should adhere well to the metal base, be flexible and Withstand a certain degree of abrasion.

Insulating coatings meeting the described requirements 1 7 ma more or less satisfactory degree have been developed by several inventors, and are described inthe patent and technical literature. Of these, two particular processes are cited here which were believed to meet most satisfactorily the requirements set forth above.

The process described in U. S. Patent 2,501,846 is essentially the creation of a glass like coating on the surface of silicon iron alloys by selectively oxidizing metallic silicon in the surface and forming a silicate by a solid 'state reaction between the oxide of thesilicon and magnesium oxide or possibly other oxides which are in contact with the surface at the timegof the treat.- ment. The process results in thin and tightly adhering films having resistivity values above 100v ohms.-/s q. cm., or evenlabove 1,000 ohms./sq. cm. Thethicknessof the coating-is estimated to be between 1 and '.1 micron,

a tenth mil in thickness.

between 40 and 4 millionths of one inch. This coating is used, extensively in commercial production with great success. Its main shortcoming is that it can only be used on silicon iron, and therefore is not applicable tonickeliron alloys. Another drawback is the rather limited temperature stability of the coating which rules out its use for cores which require final annealing treatments above 800 C. f;

The other process is described in an article by H. L.;

' B. Gould in Electrical Engineering vol. 59, 1950, pp

544548. It consists essentially of the cataphoretic deposition of colloidal silicic acid from a suspension in acetone. .The resulting film is said to bejabout one- For tapes of a thickness approaching that of the film, the resulting space. factors become very low, particularly since the tape is coated on both sides. Data on the surface resistance of the coating are lacking, and only the resistance of the core a measured from the inside of the spirally wound ring to cent of this value upon anneal is satisfactory in most area occupied by the magnetic material remain as high a as possible. This makes it necessary to reduce the thickness of the insulating film to the absolute minimum at which a uniformly insulating layer between the laminations can be'obtained. A further requirement is that these insulating layers remain unaffected bythe final- -ancases; This statement shows that a considerable change in the film resistance during the anneal must be accepted in the technique described; this fact and the relatively large thickness of the film do not recommend the technique for critical applications.

- An older method, still widely used in the actual production of wound tape cores is probably inferior in both the thickness of the film and the uniformity of insulation, but superior in the stability of the insulation during anneal. It consists of depositing a layer of magnesium oxide powder on the magnetic tape either by dusting-it with dry powder or by applying a suspension 'in methanol, to the tape by spraying, dipping, or any other suitable means. Regardless of the technique in which the magnesium powder is applied, however, the filmthick= mass produced is too thick due to the particle size and clustering of the powder and it is extremely difficult to produce a uniform film which, in turn, makes it neces- Sary to increase its thickness in order to assure a sufficient minimum level of insulation.

It is, therefore, an object of the present invention to provide a method for forming a thin coating of magnesium oxide which is adherent to the underlying surface being coated.

It is a further object to provide a method for forming a magnesium oxide coating wherein the thickness of said coating can be accurately controlled.

Another object is to provide a method for forming a magnesium oxide coating which has high resistivity to the passage of an electric current.

Another object is to provide a method for forming a magnesium oxide coating which will withstand annealing temperatures greater than 1200 C.

In accordance with the present invention, there is provided a method of forming a thin film of magnesium oxide upon a surface comprising applying a solution of magnesium methylate in an organic solvent to the surface and thereafter heating the surface to which the solution has been applied to evaporate the volatile solvent and to provide an adherent [film of magnesium oxide remaining on the surface. The solution is prepared by reacting magnesium with methyl alcohol to form magnesium methylate which reacts with moisture present to precipitate out' magnesium oxide. The resulting supernatant liquid which is a solution of magnesium methylate in methyl alcohol is diluted with an organic solvent miscible with methyl alcohol.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description and its scope will be pointed out in the appended claims.

In carrying out the present invention, magnesium metal is reacted with methyl alcohol in a ratio of about 1 part of magnesium to 50 to 75 parts chemically pure methyl alcohol. Other alcohols such as lower order aliphatic alcohols which have reactions with magnesium analogous to that of methyl alcohol may be used. The magnesium and alcohol are placed in a flask equipped with a reflux condenser and a drying tube. The reaction is initiated by seeding the solution with a crystal of iodine and applying slight heat. The application of heat is only necessary at the commencement of the reaction as the reaction will continue by itself without any external source of heat.

In the above reaction, it is not necessary to use dry reactants nor is the use of a drying tube strictly necessary as the reaction product which forms, magnesium methylate or magnesium alcoholate will react with any traces of moisture present in the system by precipitating out as magnesium oxide.

The supernatant liquid resulting from the precipitation of magnesium oxide in this reaction is a solution of magnesium methylate in methyl alcohol. This supernatant liquid is decanted and diluted 50% to 75% with methyl alcohol appreciably further lowering the concentration of the magnesium methylate in solution. It has been found that there is a relationship between the extent of the di-lution of the magnesium methylate solution and the adherence of the film, the adherence being greater if the solution is more dilute. The diluent need not be methyl alcohol but may be any organic liquid miscible with methyl alcohol such as other alcohols, ketones, etc.

The resulting solution may be applied to a surface by conventional methods such as dipping or spraying. Heating a surface to which this solution has been applied or drying it in warm air will evaporate the solvent and other volatile matter and a thin adherent film of magnesium oxide will be formed upon the surface. When the applied surface is heated, in addition to the evaporation of the solvent and other volatile matters, the magnesium methylate either because of a reaction with moisture in the air or because of heating, breaks down to magnesium oxide and a volatile substance, probably methyl alcohol. This method may be used in forming an adherent film of magnesium oxide on materials such as metals, glass, and plastic. The film may be heated up to relatively high temperatures with no change of composition, and possesses high resistivity to the passage of an electric current. The thickness of the film obtained depends upon the extent of dilution of the magnesium oxide solution and the number of times the surface to be coated is sprayed with or dipped into the magnesium methylate solution.

The new method of insulation described in the above paragraphs constitutes a considerable improvement over the prior art processes. It is superior to the glass-like coating on silicon iron in that it can withstand annealing temperatures of 1000 to 1200 C. and probably higher, and in that its application is not limited to silicon iron as a base metal. It is superior to the process using colloidal silicic acid cataphoretically deposited from a suspension in acetone in that it results in a much thinner coating and does not change its resistance excessively upon anneal. It possesses demonstrably the best combination of characteristics for this and other purposes that has yet been reported in the patent or technical literature.

The superior performance of films produced according to the new method as compared to those achieved by previous methods is due to an entirely novel way to produce the tfilm in the new method. The insulating medium in the new method is magnesium-oxide which has been known to be the most satisfactory material for this purpose for many years, because it is readily available in pure form and highly stable at high temperatures and in reducing atmospheres. The shortcoming of this material, namely, the thickness of the coating required to assure a suflicient minimum insulation, has been overcome in the new method by eliminating the use of dry or suspended particles.

In the following tables, there is provided compiled data showing the insulation efiiciency and space factor, magnetic properties, resistance, and breakdown voltage respectively of the coating which is the present invention when it is applied to magnetic tape.

TABLE I Insulation efliciency and space factor space Insulation Efficiency i Tape Size Method of Factor,

Application percent Before After Annealing Annealing 90. 6 62 35 93. 6 45 90. 3 .90 .53 I 90. 7 63 33 D11) (double) 83.6 995 87 RT =resistance of tape when wound. R =rcsistance of tape when stretched out.

TABLE II Magnetic properties D. C. A. C. (60) Tape Method of Size Application Ba Ba (25 ET H0 (25 Br Hc oerst.) oerst.)

2 mll Spray 15,350 14, 000 159 15, 200 13, 800 .320 2 mil." Dip (single)--- 2 mil. Dip (d0ub1e) 15, 600 14, 300 164 15, 250 14, 200 .320 1 mil..- D p (single)... 15, 700 750 151 13, 450 30 1 mil-.. Dlp (double). 15, 650 13, 850 .162 15, 200 13, 800 .28

Ba=flux density at saturation. Br=flux density at retentivity.

Hc=coercivity.

ins-

TABLE Ill Resistance [Electrode contact area-1 cmfl; pressure-50 111411131 Sample No. Coating Resistance, Ohms 1 Single dip-- 8.5. 2 rln 7, a Sin 6. 4 do 9.5. 5 Double 50. 6 dn 40, 7 35 R do 40. 9 Tr pl 350 to 3,000. in do 1,500 to 3,000. 11 Quadruple-...- Infinity. 12 do Do.

TABLE IV Breakdown voltage Sample No. No. Goat- Resistance, Ohms Voltage R (after ings test), ohms 2 2 2 22 2 60 2. 2 3 1,500 to Infinity 31 6.5. 4 Infinity 40 4 500 46 40.

While there has been described What is at present considered to be the preferred embodiment of the invention, it Will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting magnesium with a dry alcohol selected from the group consisting of methyl alcohol, ethyl alcohol and propyl alcohol to form the alcohol solution of the corresponding magnesium alkoxide, applying said solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

2. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting magnesium with dry methyl alcohol to form a methyl alcohol solution of magnesium methylate, applying said solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

3. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting magnesium with dry methyl alcohol to form a methyl alcohol solution of magnesium methylate, diluting said solution with an organic solvent miscible with said methyl alcohol, applying said diluted solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

4. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting magnesium with dry methyl alcohol to form a methyl alcohol solution of magnesium methylate, diluting said solution with an excess of methyl alcohol, applying said diluted solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

5. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting magnesium with an alcohol selected from the group consisting of methyl alcohol, ethyl alcohol and propyl alcohol to form a'reaction mixture'comprising said alcohol, the corre-- sponding magnesium alkoxide' and water of solution, letting said magnesium alkoxide react with said water thereby 'drying said mixture and precipitating magnesium oxide leaving a supernatant liquid comprising a dry alcohol solution of said magnesium alkoxide, decanting said solution and diluting it with an organic solvent miscible with said alcohol, applying said diluted solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface. a

6. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting magnesium with methyl alcohol to form a reaction mixture comprising methyl alcohol, magnesium methylate, and water of solution, letting said magnesium methylate react with said water thereby drying said mixture and precipitating magnesium oxide leaving a supernatant liquid comprising a dry methyl alcohol solution of said magnesium methylate, decanting said solution and diluting it with an organic solvent miscible with said methyl alcohol, applying said diluted solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

7. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting magnesium with methyl alcohol to form a reaction mixture comprising said methyl alcohol, magnesium methylate, and water of solution, letting said magnesium methylate react with said water thereby drying said mixture and precipitating magnesium oxide leaving a supernatant liquid comprising a dry methyl alcohol solution of said magnesium methylate, decanting said solution and diluting it with an excess of methyl alcohol, applying said diluted solution to said surface and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

8. A method of forming an adherent film of magnesium oxide upon a surface comprising reacting about 1 part by weight of magnesium with 50 to 75 parts by Weight of methyl alcohol to form a reaction mixture comprising said methyl alcohol, magnesium methylate and water of solution, letting said magnesium methylate react with said water thereby drying said mixture and precipitating magnesium oxide leaving a supernatant liquid comprising a dry methyl alcohol solution of said magnesium methylate, decanting said solution and diluting it with an excess of methyl alcohol, applying said diluted solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

9. In the method of forming a thin film of magnesium oxide upon a surface wherein said film is characterized by uniform, relatively high resistivity and stability at annealing temperatures up to about 1200 C., the steps comprising reacting about 1 part by Weight of magnesium with 50 to 75 parts by weight of methyl alcohol to form a reaction mixture comprising said methyl alcohol, magnesium methylate, and Water of solution, letting said magnesium methylate react with said water thereby drying said mixture and precipitating magnesium oxide leaving a supernatant liquid comprising a dry methyl alcohol solution of said magnesium methylate, decanting said solution and diluting it with an excess of methyl alcohol, applying said diluted solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

10. In the method of forming a thin film of magnesium oxide upon a surface wherein said film is characterized by uniform, relatively high resistivity and stability at annealing temperatures up to about 1200 C., the steps comprising reacting about 1 part by weight of magnesium with 50 to 75 parts by weight of methyl alcohol to form a reaction mixture comprising said methyl alcohol,--magnesium 'methylate, and water of solution, letting said magnesium methylate react With said Water thereby drying said mixture and precipitating magnesium oxide leaving a supernatant liquid comprising a dry methyl alcohol solution of said magnesium methylate, decanting said solution and diluting it 50 to 75% with an excess of methyl alcohol, applying said diluted solution to said surface, and heating said applied surface thereby evaporating all volatile matter and providing an adherent film of magnesium oxide remaining upon said surface.

References Cited in the file of this patent UNITED STATES PATENTS Lathrop July 12, 1932 Elsey Aug. 4, 1942 Byrns et a1 Dec. 13, 1949 Berringer Oct. 30, 1951 Chatfield Oct. 7, 1952' Williams et al May 23, 1954 FOREIGN PATENTS Australia Sept. 12, 1947 

1. A METHOD OF FORMING AN ADHERENT FILM OF MAGNESIUM OXIDE UPON A SURFACE COMPRISING REACTING MAGNESIUM WITH A DRY ALCOHOL SELECTED FROM THE GROUP CONSISTING OF METHYL ALCOHOL, ETHYL ALCOHOL AND PROPYL ALCOHOL TO FORM THE ALCOHOL SOLUTION OF THE CORRESPONDING MAGNESIUM ALKOXIDE, APPLYING SAID SOLUTION TO SAID SURFACE, AND HEATING SAID APPLIED SURFACE THEREBY EVAPORATING ALL VOLATILE MATTER AND PROVIDING AN ADHERENT FILM OF MAGNESIUM OXIDE REMAINING UPON SAID SURFACE. 